CN110343974B - The specific strength-elongation product is higher than 6.0 GPa%/gcm-3High-carbon low-alloy low-density steel and heat treatment process thereof - Google Patents
The specific strength-elongation product is higher than 6.0 GPa%/gcm-3High-carbon low-alloy low-density steel and heat treatment process thereof Download PDFInfo
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- C—CHEMISTRY; METALLURGY
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C21D2211/001—Austenite
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Abstract
The invention provides a composite material with a specific strength-elongation product higher than 6.0 GPa%/gcm‑3The steel comprises the following specific chemical components in percentage by mass: c: 0.55-0.75, Cr: 0.20 to 1.00, 1.0 to 2.0 Mn, 2.5 to 4.5 Al, Nb: 0.02 to 0.06, P<0.003，S<0.001 and the balance iron. The corresponding heat treatment process adopts a quenching-distribution-tempering (Q-P-T) process. The invention improves the mechanical property of the high-carbon steel by changing the chemical components of the high-carbon low-alloy steel and adjusting the heat treatment process, so that the specific strength-elongation product is higher than 6.0 GPa%/gcm‑3The above.
Description
Technical Field
The invention relates to a chemical composition and a heat treatment process of steel, namely a specific strength-elongation product higher than 6.0 GPa%/gcm-3High carbon low alloy low density steel and its heat treatment process.
Background
The choice of automotive material is governed by several factors, the three most important of which are: lightweight, weldability and safety. The development of Advanced High Strength Steels (AHSSs) meets the requirements of reducing the weight of automobiles, achieving the purposes of energy conservation and emission reduction, and also enhancing the safety. The low-density steel developed on the basis of AHSSs research can further reduce the weight of the automobile and more effectively meet the aims of energy conservation and emission reduction. At present, low-density steel is developed mainly by adding Al on the basis of the third generation advanced high-strength steel.
The basic mechanical properties of steel materials are generally characterized by strength (unit: MPa) and elongation (unit:%; may be used to indicate plasticity). The strength and plasticity of steel materials are often contradictory, i.e., an increase in the strength of a material results in a decrease in its plasticity. Therefore, in order to comprehensively determine the material properties, the product of tensile strength and total elongation (PSE) of the material is usually used as the criterion of comprehensive properties. If the lightweight property is also brought into the standard of evaluating the comprehensive mechanical property of the steel, the composite material is more suitable than a product of strength and elongation (SPSE: PSE/density), namely the higher the product of strength and elongation of unit density is, the lighter the weight of the automobile structural part is under the condition that the strength and the ductility of the steel material are not reduced, and the effects of energy conservation and emission reduction can be achieved. Low-density steels can be broadly classified into three categories according to alloy composition and room-temperature microstructure: 1) ferritic steel; 2) a ferritic/austenitic dual-phase steel; 3) an austenitic steel. The three types of steel basically correspond to three generations of advanced high-strength steel, and only the carbon content is increased to make up for the reduction of the strength after Al is added. Therefore, the Al-containing high-carbon low-alloy steel has a greater potential than the product of strength and elongation than the Al-containing medium-carbon and low-carbon low-alloy steels. The high specific strength-product indicates that the light material has excellent comprehensive mechanical properties, and the obtaining way is to carry out optimal material component design and heat treatment process design. The high specific product of strength and elongation is an important index of advanced high-strength steel.
The prior art documents are searched to find that:
Yi H L,Ryu J H,Bhadeshia H K D H,et al.Low-alloy duplex,directly quenched transformation-induced plasticity steel[J]script materials 2011, 65(7):604-607, the article carries out heat treatment with different processes on 0.4C-1.03Mn-2.95Al-0.22Si light steel to obtain a mixed structure of delta-ferrite, retained austenite and martensite. The result shows that the best performance is the strength of 900MPa and the elongation of 28 percent, the highest product of strength and elongation is 25.2GPa percent, and the specific product of strength and elongation is 3.3GPa percent/gcm-3. Furthermore, studies of the reddish brilliance and the like have also shown that: an alloy of 2.50 wt.% Al (Fe-0.4C-0.26Si-2.02Mn-2.50Al, wt.%) austenitizes when heated to 1000 ℃, i.e., the high temperature delta-ferrite is fully transformed into austenite, and thus the hot rolled structure of the 2.50 wt.% Al steel is an alpha ferrite + pearlite structure that is conventionally transformed from austenite. Part of the high temperature delta-ferrite remains after the alloy with an increased Al content of 3.50 wt.% (Fe-0.39C-0.77Si-1.50Mn-3.35Al, wt.%) is heated to 1000 ℃. The low-alloy delta-ferrite low-density steel is reported to be medium-carbon steel and low-carbon steel at present, the product of strength and elongation is 20-30 GPa%, and the density is 7.5g/cm3About the same, therefore, the SPSE is 2.7-4.0 GPa%/gcm-3。
Sohn S S,Lee B J,Lee S,et al.Effect of annealing temperature on microstructural modification and tensile properties in 0.35C-3.5Mn-5.8Al lightweight steel[J]Acta Materialia,2013,61(13):5050-5066. the article carries out heat treatment on 0.35C-3.5 Mn-5.8Al light steel by different processes to obtain a mixed structure of delta-ferrite, retained austenite and alpha-ferrite. Test results show that the sample obtains the highest strength and plasticity after being heated and insulated for 50s at 880 ℃ and isothermally treated for 180s at 400 ℃, the best performance is the strength of 800MPa and the elongation of 42 percent, the highest product of strength and elongation is 33.6GPa percent, and the specific product of strength and elongation is 4.5GPa percent/gcm-3. It is noted that the alloy component of the steel grade is 2-3 times higher than that of the material, Mn is an austenitizing element, and the addition of Mn not only can improve the austenite content and enlarge the range of an austenite phase region, but also can enable austenite to be formed at a lower temperature and accelerate the growth rate of austenite grains, so that the delta-ferrite is crushed and distributed, and the purpose of improving plasticity is achieved. This aluminum-rich medium manganese steel has not been in the low alloy steel range.
Suh D W,Park S J,Lee T H,et al.Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon,Manganese Transformation-Induced-Plasticity Steel[J].Metallurgical&Materials transformations A,2010,41(2):397-408, the article carries out heat treatment on 0.12C-5.8Mn-3.1Al-0.47Si light steel by different processes to obtain a mixed structure of delta-ferrite, retained austenite and alpha-ferrite. The result shows that the best performance is 994MPa of strength and 27 percent of elongation, the highest product of strength and elongation is 26.8GPa percent, and the specific product of strength and elongation is 3.48GPa percent/gcm-3。
The article published by Dongfeng et al, which is about the influence of annealing temperature on the structure and mechanical properties of ferrite-based light steel, Shanghai metals, 2017, 39(04):5-9. A ferrite-based light steel with Fe-0.25C-3.5Mn-8Al as a component (mass fraction percent) is designed, and the relation between the microstructure and the mechanical property of the test steel under different heat treatment conditions is researched. The result shows that the content of the retained austenite in the sample is continuously increased along with the increase of the annealing temperature, the tensile strength and the elongation after fracture are in a gradually increasing trend, after the treatment of heat preservation at 950 ℃ for 50s and finally heat preservation at 400 ℃ for 3min, the product of strength and elongation of the test steel reaches the maximum value of 22.451 GPa%,the maximum ratio of strength to elongation product can reach 3.13 GPa%/gcm-3。
The research above all achieves the purpose of improving the performance of steel by controlling the components of light steel and changing the heat treatment process without exception, but the steel is medium-low carbon light alloy steel, and the obtained specific strength-to-strength product does not exceed 4.5 GPa%/gcm-3. The material is high-carbon low-alloy steel containing Al, the alloy content and the cost are reduced, the performance can be obviously improved by adjusting the heat treatment process, and the specific strength-elongation products obtained in the invention are all higher than 6.0 GPa%/gcm-3The difference from the above search literature results is very obvious. This shows that under the condition of similar contents of carbon, silicon and manganese, the performance of the high-carbon steel still has a very large improvement space by changing the alloy element proportion and the heat treatment process.
Disclosure of Invention
At present, the research on light steel is mainly focused on medium-low carbon alloy steel, the performance of the medium-low carbon alloy steel is improved by controlling the components of the light steel and changing the heat treatment process, but the obtained specific strength product does not exceed 4.5 GPa%/gcm-3. Aiming at the situation, the invention provides a specific product of strength and elongation higher than 6.0 GPa%/gcm-3High carbon low alloy low density steel and its heat treatment process.
According to a first aspect of the present invention, there is provided a specific product of strength and elongation higher than 6.0 GPa%/gcm-3The high-carbon low-alloy low-density steel of (a) has the following specific components (mass fraction):
c: 0.55 to 0.75, Al 2.5 to 4.5, Mn 1.0 to 2.0, Cr: 0.20 to 1.00, Nb: 0.02-0.06, P <0.003, S <0.001, and the rest is iron.
The preferable ranges of the components of the high-carbon low-alloy low-density steel are as follows (mass fraction,%):
c: 0.62 to 0.69, 3.5 to 4.5 Al, 1.3 to 1.8 Mn, Cr: 0.40 to 1.00, Nb: 0.04-0.06 wt%, P <0.003 wt%, S <0.001 wt%, and the rest is Fe.
The design principle of the chemical components comprises the following steps: c can reduce the density of the steel most effectively, and the high C content can obviously reduce the density of the steel compared with the medium-low C content; meanwhile, the high C content can effectively improve the strength of the material and make up for the reduction of the strength caused by the addition of Al; the C content is in direct proportion to the austenite stability and in inverse proportion to Ms. Mn is an austenitizing element, and the addition of the Mn element can expand an austenite phase region, improve the austenite content, improve the fault energy of steel, inhibit martensite phase transformation, generate dense twin crystals in the deformation process and effectively improve the elongation of the steel. The addition of Cr mainly improves the hardenability of steel and has a solid solution strengthening effect; al is a ferrite strong stabilizing element, which can reduce the density of steel, improve the stacking fault energy, improve the ferrite stability and inhibit the formation of austenite, so that delta ferrite can exist stably, and an austenite phase region is extruded and moves towards the direction of increasing the content of C and Mn. Nb can effectively refine austenite grains in the austenitizing treatment, thereby generating a fine grain strengthening effect; meanwhile, the alloy can form stable Nb carbide with C, thereby generating dispersion strengthening and finally improving the yield strength and the toughness of the steel.
According to another aspect of the present invention, there is provided a heat treatment process of the above steel, which requires: the heat treatment adopting a specific quenching-distribution-tempering (Q-P-T) process comprises the following specific steps:
Q-P-T process: austenitizing temperature in a two-phase region: 880-960 ℃, the heat preservation time is not easy to be overlong so as to prevent coarse grains, and then the steel is quenched to the martensite phase transformation starting temperature MsAnd an end temperature MfAt a certain temperature T in betweenq: 230-330 ℃, then completely immersing the sample into a salt bath with the tempering temperature of 330-450 ℃ for heat preservation, wherein the heat preservation time is determined according to the section size of the sample, and finally taking out the sample and quenching the sample in water to room temperature.
The design principle of the heat treatment process of the invention is as follows:
in the Q-P-T process, because of the addition of alloy elements, the austenitizing heat preservation is actually heating in a ferrite and austenite two-phase region, and the austenitizing temperature is selected for adjusting the organization and the content of the steel, so that not only is carbide completely dissolved in austenite to form uniformly distributed austenite and ferrite organizations, but also the relative content of the austenite and the ferrite is adjusted to obtain the optimal mechanical property. The austenite holding time is selected to preserve the grainsIs fine. In the Q-P-T process, the quenching temperature (T)q) Is selected to adjust the content of austenite that changes to martensite during quenching; and in combination with carbon distribution during tempering, carbon is diffused from supersaturated martensite to adjacent retained austenite, and the stability of the retained austenite is increased. The selection of the carbon diffusion time and temperature in the tempering process is determined according to the carbon distribution degree and the carbide formation, and in the process, due to the reduction of the carbon content of martensite, the lattice distortion of the martensite is reduced, the dislocation density is obviously reduced, so that the deformation capacity of the martensite is increased, and the sample can obtain higher strength and plasticity.
Steel obtained by the above treatment: the volume fraction of the steel obtained after the heat treatment is as follows: 35-45% of delta-ferrite, 25-35% of retained austenite, 25-35% of martensite and stable Nb carbide (less than 1%); the strength is 1100-1400 MPa, the elongation is 40-32%, the product of strength and elongation is 42-48 GPa%, and the ratio of product of strength and elongation (SPSE) is up to 6.0 GPa%/gcm-3The above.
Compared with the prior art, the invention has the following beneficial effects:
the component design and heat treatment process method can obtain the product of strength and elongation of more than 6.0 GPa%/gcm-3The high-carbon low-alloy low-density steel. Compared with ferrite/austenite dual-phase steel and austenite steel, the content of Mn and Al elements in the steel is greatly reduced, and the cost is obviously reduced. The Mn content is only one twentieth to one tenth of that of the austenitic steel, but the tensile strength of the steel is much higher than that of the high-Mn austenitic steel, such as Fe-15Mn-0.8C-8.5Al-1.5Si steel, and is only 870 MPa. The high carbon and low alloying of the invention has a specific product of strength and elongation higher than 6.0 GPa%/gcm-3The high-carbon low-alloy low-density steel with high strength and high plasticity is not reported at home and abroad at present.
The components and the heat treatment process are suitable for various mechanical structural parts, forgings or rolled pieces.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.
Example 1
(1) The specific composition of the steel in this example is as follows (mass fraction,%):
c: 0.67, Mn: 1.50, Al: 4.00, Cr: 0.60, Nb: 0.04, P: 0.0029, S: 0.001, the remainder being iron;
(2) the specific heat treatment process of the invention is as follows:
the piece to be treated was a hot-rolled plate 15mm thick.
And (3) carrying out heat treatment of quenching-distributing-tempering (Q-P-T) process on the piece to be treated. The method specifically comprises the following steps:
Q-P-T process: austenitizing temperature: keeping the temperature at 950 ℃ for 300s, and then quenching to: 290 deg.C (martensite start temperature (M))s) And finishing temperature (M)f) Temperature (T) betweenq) Heat preservation for 10s, then heat preservation in a salt bath at 400 ℃, heat preservation for 600s, and finally water cooling to room temperature.
And (3) processing results:
and (3) microstructure: delta-ferrite (volume fraction: 40%), martensite (volume fraction: 31.6%), retained austenite (28%), Nb carbide (less than 1%).
1240MPa of strength, 37 percent of elongation and 45.756GPa percent of product of strength and elongation, and 6.183GPa percent/gcm percent of product of specific strength and elongation-3。
Example 2
(1) The specific composition of the steel in this example is as follows (mass fraction,%):
c: 0.65, Mn: 1.20, Al: 3.50, Cr: 0.30, Nb: 0.04, P: 0.0028, S: 0.001, the remainder being iron;
(2) the specific heat treatment process of the invention is as follows:
the piece to be treated was a hot-rolled plate 15mm thick.
And (3) carrying out heat treatment of quenching-distributing-tempering (Q-P-T) process on the piece to be treated. The method specifically comprises the following steps:
Q-P-T process: ao nationality of OlympicTemperature of the Kirschner wires: keeping the temperature at 940 ℃ for 300s, and then quenching to: 270 deg.C (martensite start temperature (M))s) And finishing temperature (M)f) Temperature (T) betweenq) Heat preservation for 10s, then heat preservation in a salt bath at 400 ℃, heat preservation for 600s, and finally water cooling to room temperature.
And (3) processing results:
and (3) microstructure: delta-ferrite (volume fraction: 35%), martensite (volume fraction: 34%), retained austenite (30.6%), Nb carbide (less than 1%).
The strength is 1300MPa, the elongation is 36.7 percent, the product of strength and elongation is 47.71GPa percent, and the specific product of strength and elongation is 6.447 GPa%/gcm-3。
Example 3
(1) The specific composition of the steel in this example is as follows (mass fraction,%):
c: 0.75, Mn: 1.60, Al: 4.00, Cr: 0.60, Nb: 0.06, P: 0.0029, S: 0.001, the remainder being iron;
(2) the specific heat treatment process of the invention is as follows:
the piece to be treated was a hot-rolled plate 15mm thick.
And (3) carrying out heat treatment of quenching-distributing-tempering (Q-P-T) process on the piece to be treated. The method specifically comprises the following steps:
Q-P-T process: austenitizing temperature: keeping the temperature at 950 ℃ for 300s, and then quenching to: 280 deg.C (martensite start temperature (M))s) And finishing temperature (M)f) Temperature (T) betweenq) Heat preservation for 10s, then heat preservation in a salt bath at 400 ℃, heat preservation for 600s, and finally water cooling to room temperature.
And (3) processing results:
and (3) microstructure: delta-ferrite (volume fraction: 39%), martensite (volume fraction: 30%), retained austenite (30.6%), Nb carbides (less than 1%).
1240MPa of strength, 38.4 percent of elongation, 47.616GPa percent of product of strength and elongation, and 6.435GPa percent/gcm of specific product of strength and elongation-3。
The above specific embodiments have described the present invention in detail. It should be noted that the components and the heat treatment process involved in the present invention are not limited to the specific manner described above, and those skilled in the art can make various changes or modifications within the scope of the claims without affecting the essence of the present invention.
Claims (2)
1. The specific strength-elongation product is higher than 6.0 GPa%/gcm-3The high-carbon low-alloy low-density steel is characterized by comprising the following chemical components in percentage by mass: c: 0.65 to 0.75, Al 3.5 to 4, Mn 1.2 to 1.6, Cr: 0.2 to 0.6, Nb: 0.04 to 0.06, P<0.003, S.ltoreq.0.001, the remainder being iron, obtained by: the heat treatment adopting a specific quenching-distributing-tempering process comprises the following specific processes: austenitizing temperature in a two-phase region: keeping the temperature for 300s at 880-960 ℃, and then quenching to the martensite phase transformation starting temperature Ms and the martensite phase transformation finishing temperature MfA certain temperature Tq therebetween: the temperature is 230-330 ℃, the temperature is kept for 10s, then the sample is completely immersed into a salt bath with the tempering temperature of 330-450 ℃ again, the temperature is kept for 600s, the temperature keeping time is determined according to the section size of the sample, and finally the sample is taken out and quenched in water to room temperature; the volume fraction of the steel obtained after the heat treatment is as follows: 35-45% of delta-ferrite, 25-35% of retained austenite, 25-35% of martensite and less than 1% of stable Nb carbide; the tensile strength is 1100-1400 MPa, the elongation is 40-32%, the product of strength and elongation is 42-48 GPa%, and the specific product of strength and elongation is 6.0 GPa%/gcm-3The above.
2. The specific strength-elongation product is higher than 6.0 GPa%/gcm-3The heat treatment process of the high-carbon low-alloy low-density steel is characterized by comprising the following steps of: the steel comprises the following chemical components in percentage by mass: c: 0.65 to 0.75, Al 3.5 to 4, Mn 1.2 to 1.6, Cr: 0.2 to 0.6, Nb: 0.04 to 0.06, P<0.003, S is less than or equal to 0.001, the rest is iron, and the specific heat treatment process of quenching-distributing-tempering process is as follows: austenitizing temperature in a two-phase region: keeping the temperature for 300s at 880-960 ℃, and then quenching to the martensite phase transformation starting temperature Ms and the martensite phase transformation finishing temperature MfA certain temperature Tq therebetween: 230-330 ℃ and preserving heat for 10s, then completely immersing the sample into a salt bath with the tempering temperature of 330-450 ℃ again and preserving heat for 600s, wherein the preserving heat time is determined according to the section size of the sample, and mostThen taking out the sample and quenching the sample in water to room temperature; the volume fraction of the steel obtained after the heat treatment is as follows: 35-45% of delta-ferrite, 25-35% of retained austenite, 25-35% of martensite and less than 1% of stable Nb carbide; the tensile strength is 1100-1400 MPa, the elongation is 40-32%, the product of strength and elongation is 42-48 GPa%, and the specific product of strength and elongation is 6.0 GPa%/gcm-3The above.
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